The invention discloses a method for preparing a carbon nanotube by taking cracking carbon black as a carbon source. The method mainly comprises production of cracking carbon black, pretreatment of the cracking carbon black and preparation of the nanotube. In the production of the cracking carbon black, a waste tire is subjected to thermal cracking by a specific thermal cracking tower; then, the cracking carbon black is pretreated through the processes such as screening, grinding, acid pickling and purifying and modifying to obtain a carbon nanotube raw material with good performance, and the carbon nanotube raw material is applied to the production of a carbon nanotube. Carbon nanotube production equipment can realize continuous feeding of catalysts and raw materials so as to realize continuous production and improve the quality of the carbon nanotube. Since the initial raw material for producing the carbon nanotube in the method is the waste tire, the production cost of the carbon nanotube can be effectively lowered, the waste tire is recycled as well, and thus, the method is of far-reaching significance and influence to energy conservation and emission reduction and eco-environmental protection.